In recent years the use of high-strength-to-weight ratio fiber reinforced resin composites has continuously expanded, particularly in weight-sensitive, products, such as aircraft and space vehicles. Prior to the present invention, the fiber reinforced resin composites used in such products have usually been created by forming a layup, e.g., a stack, of layers or plies, the layers or plies being formed of unidirectional or multidirectional (e.g., woven) fabrics made of glass or graphite fibers impregnated with a resin. Such preimpregnated plies are commonly referred to as "prepreg" plies or simply prepreg. Normally the layup is positioned atop a forming tool, which, in its simpliest form, may comprise a flat plate. After the layup is prepared, heat and pressure are applied. The heat cures the resin and the pressure compresses the layup preventing air and other gases, including volatile gases, from forming porosity (bubbles) as the resin cures. Normally an autoclave is used to apply the necessary heat and pressure.
While monolithic structures formed of fiber reinforced resin composites processed in the manner described above are satisfactory in some environments, they have certain disadvantages. For example, it has become desirable to provide cross-ply reinforcement in order to increase resistance to "in-plane" compression load failure, particularly after limited input damage. In-plane loads are those lying in the plane of the plies. Cross-ply reinforcement (sometimes referred to as Z-direction reinforcement) is created by cross-ply stitching a layup. While this idea is uncomplicated in theory, in the past its implementation has been difficult. The difficulty occurs because the plies are preimpregnated with resin, which is sticky. The sticky resin makes it extremely difficult to cross-ply stitch the fiber layers together.
Another disadvantage of using fiber plies preimpregnated with resin is the difficulty of removing gases trapped between the plies when a layup is formed and the gases produced in the layup when the resin is being cured. While the pressure applied during curing forces most entrapped gases into solution (in the same way water is prevented from boiling at elevated pressure), some bubbles still form, resulting in weakening voids in the resultant monolithic structure.
A further disadvantage associated with the use of preimpregnated fiber plies is the need to store such plies at a low temperature and the losses associated with the failure to use such plies in a timely manner. More specifically, as will readily be appreciated by those familiar with resins used to create fiber reinforced resin composites, the rate of resin curing is accelerated when resin temperature is raised. Conversely, the rate of resin curing is retarded by low temperatures. As a result, conventionally, prior to use, preimpregnated fiber reinforced layers (which are usually in the form of relatively wide tape or fabric on rolls prior to being laid up) are stored in a refrigerated environment. Since the low storage temperature impedes resin curing, the usable life of prepreg is increased. The disadvantage of this technique is that because the fiber and resin form a unitary structure, the fiber, as well as the resin, must be stored. As a result, a relatively large amount of storage capacity is required. A further disadvantage is that even at low temperatures resin cures, albeit at a slower rate. As a result, at some point, even prepreg stored at low temperature become unusable and must be disposed of. That is, even though the resin is the only portion of the prepreg that becomes useless, the fiber as well as the resin must be disposed of because the resin is impregnated into the fiber.
The invention is directed to avoiding the disadvantages of creating monolithic structures from preimpregnated fibrous layers. More specifically, the invention is directed to providing a method and apparatus for creating fiber reinforced resin composites that can be easily cross-ply stitched and are formed in a manner that substantially reduces if not entirely eliminates weakening voids created by trapped gases by removing such gases prior to and during the infusion of the resin. Further, the invention is directed to providing a method and apparatus suitable for creating fiber reinforced resin composites that substantially reduces the amount of waste resulting from the curing of stored resins and the rejection of prepreg material due to poor quality.